Epoxy resins are commercially important materials that are used extensively to make thermosetting products for use in coatings, adhesives, composites, and many other applications. The largest volume of epoxy resins utilized in commerce are those based upon the diglycidyl ether of bisphenol-F (DGEBF), epoxy novolac resins, and those based upon the diglycidyl ether of bisphenol-A (DGEBA). Of these, the bisphenol-A based products are utilized in much larger volumes than the other products.
Despite the fact that epoxy resins can be crosslinked with amino resins and the like through the secondary hydroxyl groups on the resin backbone, it is generally found that significantly higher temperatures and/or bake times are required than are necessary with other polyols utilized in coatings, such as acrylic polyols and polyester polyols. It is thought the relatively hindered environment of the OH groups on the epoxy resin is responsible for this effect. Obviously, this is usually a significant drawback to the utilization of epoxy resins in such coatings, since higher oven temperatures and/or bake times lead to higher production costs.
Cationic or acid-catalyzed polymerization (or homopolymerization) of multifunctional epoxy resins in bulk or in organic solvent solution is a well-known process of significant commercial importance. Lewis acids are most commonly employed, but appropriate Bronsted acids can also be utilized. For example, C. A. May (Ed.), Epoxy Resins Chemistry and Technology, Marcel Dekker, Inc.: New York, 1988, reports (p. 495) that Lidarik et. al. (Polymer Sci. USSR, 1984, 5, 589) polymerized glycidyl ethers with complexes of antimony pentachloride, boron trifluoride, and perchloric acid. Additional examples are reported in May. In addition, the photoinitiated cationic polymerization of epoxy resins is well-known, and also of commercial importance. As reviewed in May (pp. 496-498), cationic photoinitiators are materials that upon photolysis generate strong Bronsted acids, which serve as the true catalyst for the epoxide polymerization.
The general structure of resins obtained by polymerization of multifunctional epoxides differs from the linear structure of a bisphenol-A resin. In the cationic polymerization process, an alcohol or water serves as an initiator for polymerization, which occurs by attack onto an activated (protonated or coordinated with a Lewis acid) epoxide. Note that the alcohol may be one of the hydroxyl groups already present on an epoxy resin backbone. The product that results is an alcohol, that can go on to react with another protonated epoxide. Note also that attack can occur at the unsubstituted carbon on the epoxide ring to yield the secondary alcohol, or on the substituted carbon to yield the primary alcohol. This is in contrast to the epoxy advancement process used to make higher molecular weight bisphenol-A resins, which is conducted under conditions where the secondary alcohol is formed almost exclusively. ##STR1##
Since this process can occur at both ends of a difunctional resin, it should be clear that even with small degrees of reaction a highly branched material will be formed, which will quickly reach the gel point and yield a crosslinked, intractable material. Thus, acid catalyzed polymerizations are very useful for forming highly crosslinked final products like coatings and composites. However, the acid catalyzed polymerization of a difunctional or higher functional epoxy resin is inherently a difficult process to control, and acid catalyzed polymerizations are not broadly employed to produce higher molecular weight but still tractable products that can then be formulated and crosslinked with another resin such as an amine hardener or a melamine-formaldehyde resin to form a final product.
Due to increasing concerns about the negative effects of organic solvents on the environment and human health, there is a need in many industries, including the paint and coatings industry, for products with reduced levels of volatile organic content (VOC). There are many approaches to reducing VOC in the coatings industry, but certainly one of the most important is to utilize waterborne (WB) coatings, where the organic solvents which had formerly been used to reduce the viscosity of the paint formulation to the point necessary for application are replaced wholly or in part with water. Of course, since water is not a solvent for most paint binders, switching to a WB formulation usually requires chemical modification of the binder resins, or changing the form of the binder by emulsifying it, or a combination of these techniques.
There are many ways in which epoxy resins have been emulsified or dispersed to make waterborne coatings, and the technology has recently been reviewed (F. H. Walker and M. I. Cook, "Two-Component Waterborne Epoxy Coatings", in J. E. Glass (ed.), Technology for Waterborne Coatings, American Chemical Society: Washington, D.C., 1997, pp. 71-93). In general, special processes and specially designed surfactants must be employed to emulsify a high molecular weight epoxy resin such as a bisphenol-A type resin due to the high viscosity of the resin.
For example, U.S. Pat. No. 4,415,682 describes the BF.sub.3 catalyzed reaction of a polyethylene glycol with liquid epoxy resin to form a block copolymer. The block copolymer is then used to emulsify a solution of a solid epoxy resin. However, to develop low VOC coatings from these epoxy resin dispersions, the solvent must be subsequently removed.
U.S. Pat. No. 4,315,044 describes reacting diglycidyl ethers of poly(ethylene oxide-co-propylene oxide), bisphenol-A, and liquid epoxy resin to prepare a surfactant in situ during the high temperature epoxy advancement process, and then dispersing the resin by adding a mixture of glycol ether solvent and water with vigorous agitation. As in the above system, substantial amounts of organic solvent are required in this process.
U.S. Pat. No. 4,608,406 modifies this process by incorporation of a novolac resin to increase the functionality of the dispersion.
The free radical process for polymerization of aqueous emulsions of unsaturated monomers, the so-called emulsion polymerization process, is widely practiced. There exist only a few references to the preparation of polysiloxanes, and hydrophobic polyesters and polyketals, by the acid catalyzed step-growth polymerization of emulsified monomers.
GB 2,303,857 describes the polymerization of emulsions of octamethylcyclo-tetrasiloxane with sulfonate surfactants.
Y. Yang and coworkers likewise studied the polymerization of emulsions of octamethylcyclotetrasiloxane (Hecheng Xiangjiao Gongye, 8(2), 100-4, 1985; Hecheng Xiangjiao Gongye, 8(1), 15-18, 1985; Gaofenzi Tongxun (2), 104-9, 1983; Gaofenzi Tongxun (4), 310-13, 1982; Gaofenzi Tongxun (4), 266-70, 1982). D. R. Weyenberg et. al. (Polym Prepr., Am. Chem. Soc., Div. Polym. Chem., 7(2), 562-8, 1966) also report the polymerization of emulsions of permethylcyclosiloxanes. One could speculate that since the equilibrium concentration of water in emulsions of polysiloxanes is very low, that polymerizations to form polysiloxanes, which are step-growth equilibrium processes involving addition and loss of water, would be ideal candidates for such a polymerization process.
U.S. Pat. No. 4,355,154 describes a method for polymerizing emulsions of hydrophobic polyacids and polyols to make polyesters using traditional polycondensation catalysts. U.S. Pat. No. 4,374,953 describe a process for making polyacetals and polyketals from emulsions of polyols and carbonyl compounds using traditional polycondensation catalysts. These are also equilibrium, or reversible, processes, requiring relatively hydrophobic monomers in order to achieve a useful degree of polymerization.
There appear to be no examples in the chemical literature of the cationic (i.e., acid catalyzed), non-reversible, or non-equilibrium, polymerization of monomers emulsified in an aqueous media, such as ring-opening polymerization of cyclic ethers. This may be because cationic polymerizations of this type are in general known to be quite subject to rapid termination reactions by trace impurities such as water. Thus, in aqueous emulsion systems, the equilibrium concentration of water in the organic phase is usually fairly high, on the order of tenths of a percent to several percent, and hence it would be anticipated that polymerization would be difficult.
U.S. Pat. No. 5,766,768 discloses two component water-based compositions prepared by reacting enhanced molecular weight epoxy emulsions with amine curatives, the enhanced molecular weight epoxy emulsions being prepared by polymerizing water-borne epoxy resin particles in the presence of an alkaline substance.